The present invention relates to a preferred pneumatic tire for use in a racing cart and to a manufacturing method thereof, more particularly, to a pneumatic tire designed so as to maximize its footprint area in an inflated state with inflation pressure (inflated state) and to prevent an expansion and growth of the outer diameter at high speed running and to a manufacturing method thereof.
A bias tire for use in a racing cart and the like generally has a low aspect ratio and a wide tread width and is set to have a cord angle of 40 xc2x0 or less of a carcass layer to a tire circumferential direction. However, differing from a radial tire, since a belt member pressing a footprint is not laid in a tread portion of a bias tire, a tread radius on a tire meridian section deforms to be small in an inflated state, whereby a footprint width tends to be small. And, when the footprint width becomes small, driving stability is lowered.
In order to control the above-described reduction of the footprint width, employing the following tread shapes has been proposed: a tread shape including its tread center portion made to cave by giving a counter radius expanding to the inside of the tire on the surface of the tread portion; or a tread shape combined with a counter radius expanding to the inside of the tire and a normal radius expanding to the outside of the tire.
However, a tread shape including the foregoing counter radius has not enabled a footprint area to be fully enlarged since the tread surface becomes uneven in an inflated state. Moreover, in a bias tire for use in a racing cart and the like, although the foregoing tread shape is employed, effects of controlling an expansion and growth of the outer surface are also insufficient since the tread portion expands to the outside of the tire diameter direction due to a centrifugal force at high speed running.
It is an object of the present invention to provide a pneumatic tire, in which a footprint area can be maximized and an expansion and growth of the outer diameter can be prevented, and to a manufacturing method thereof.
In order to achieve the foregoing object, a pneumatic tire of the present invention which has a bias ply structure in which cords are disposed so as to cross each other between plural carcass layers and has aspect ratio of 55% or less and an outer diameter of 300 mm or less, wherein a cord angle of the carcass layer with respect to the tire circumferential direction ranges 27 to 37xc2x0 at the center line position, and a surface shape of a tread portion is formed to be straight with respect to the tire width direction in a region of 60 to 75% of the tread width.
Thus, a cord angle in carcass layers ranges 27 to 37xc2x0 at the center line position, a tread portion is formed to be straight with respect to the tire width direction in a region of 60 to 75% of the tread width, and both shoulders are coupled at the shortest distance therebetween. Thereby, in an inflated state, the tread surface presses the earth uniformly without becoming uneven. Thus, the footprint area can be maximized, moreover, an expansion and growth of the outer diameter can be minimized. Therefore, a pneumatic tire of the present invention is preferable for use in racing, especially for a racing cart, and can exhibit excellent driving stability.
In the present invention, the surface shape of the tread portion is the shape of the inside of a tire forming die. This surface shape of the tread portion is preferably made to be straight with respect to the tire width direction in the foregoing region. However, the surface shape may be a circular arc having a radius of more than 900 mm, that is substantially straight. Moreover, the region (straight region) forming the tread portion to be straight is preferably disposed so that the center of the region can be on the center line. In addition, the foregoing tread width corresponds to the distance between the points of intersection described below. These points of intersection are made by crossing a virtual line extending the border of the tread portion to the outside of the tire width direction and a virtual line extending the border of the side wall portion to the outside of the tire diameter direction.
The above-described pneumatic tire can be manufactured by the method described below. That is, the method of manufacturing the pneumatic tire of the present invention is for manufacturing a pneumatic tire adopting a bias ply structure in which plural carcass layers are disposed so that cords cross each other between the layers, and having an aspect ratio of 55% or less and an outer diameter of 300 mm or less. The method comprises the steps of: forming the carcass layer to be cylindrical, forming a green tire by winding a tread rubber extrusion having a belt state round the outer periphery of the carcass layer, expanding a diameter of the green tire while housing the green tire in a die so that a cord angle of the carcass layer with respect to the circumferential direction is set to a range from 27 to 37xc2x0 at the center line position, and forming a surface shape of the tread portion formed of the tread rubber extrusion to be straight with respect to the tire width direction in the region of 60 to 75% of the tread width.